These days, in an electric power steering device, construction of a double system, for example, is demanded in order to maintain normal operations even when an abnormality happens in one of its elements. For the abnormality that happens in a torque sensor, a double system may be constructed by providing two torque sensors, but the cost is high in that case.
Therefore, there is a technique described in Patent Literature 1, for example, for constructing a double system without providing two torque sensors. In this technique, at normal time of a torque sensor, steering assistance control is carried out by using a torque sensor value, whereas at abnormal time of the torque sensor, the steering assistance control is carried out by using steering torque that is calculated by using a motor rotation angle sensor and a steering angle sensor. Here, by counting a cycle of a detection signal of the motor rotation angle sensor and a cycle of a detection signal of the steering angle sensor, respectively, absolute angles are respectively calculated, so that the steering torque is calculated based on a difference (a relative angle) between the absolute angles.